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Yes, I suppose that they would be sister-in-laws. Because your sister is your husband's sister-in-law, and the other sister is his sibling, then I suppose they would be sister-in-laws.
if my sister had a baby boy what would he be to me
The correct form is "my sister and I" when it is the subject of a sentence, and "my sister and me" when it is the object.
Your mother's sister would be your aunt. Your father's sister would also be your aunt.
your mother's sister would be your aunt
Your mother's sister's husband's sister is your aunt's sister-in-law, but is not related to you. Here's why: Your mother's sister is your aunt. The sister of a woman's husband would be her sister-in-law. In this case the woman is your aunt. So your mother's sister's husband's sister would be your aunt's sister-in-law.
The sister of your husband is your sister-in-law.
She would be your step-sister
Your sister in law is your husband's sister or your brother's wife.
Sister is a singular term, not plural, therefore you would say how is your sister, not how are your sister.
Your child would call your sister her aunt.Your sister would call your child her niece or nephew depending on gender.Your sister's children would call your child their cousin.